Following a long period of environmental neglect, the United States and other countries have placed a high priority on remediation of contaminated sites. It is estimated that between 300,000 and 400,000 contaminated sites are scheduled for cleanup in the United States in the coming decades, at an estimated cost as high as $500 billion to $1 trillion (National Research Council, “Alternatives for Ground Water Cleanup, Washington, D.C.: National Academy Press, 1994; M. Russell et al., Hazardous Waste Remediation: The Task Ahead, Knoxville: University of Tennessee, 1991). The fore-going publication, and each additional publication cited herein, is incorporated herein by reference. U.S. spending on waste site remediation totaled approximately $9 billion in 1996 alone.
Despite this considerable investment, conventional technologies for remediation of contaminated sites, especially those with contaminated ground water, are inadequate. For example, the National Research Council (NRC) has conducted a study of conventional ground water cleanup systems at 77 contaminated sites and determined that ground water cleanup goals had been achieved at only 8 of the sites and that full achievement was highly unlikely with the in-place technologies at 34 of the 77 sites (NRC, ibid., 1994; MacDonald and Kavanaugh, Envir. Sci. Tech. 28(8), 362A-68A, 1994). Based on these findings, it is believed that improved technologies are needed to restore contaminated sites.
The most common types of contaminants found at waste sites include chlorinated solvents, petroleum hydrocarbons, and metals (NRC, 1994). Chlorinated solvents, such as carbon tetrachloride, trichloroethylene, and perchloroethylene, are used for such purposes as dry cleaning and degreasing industrial manufacturing equipment and cleaning military aircraft. Petroleum hydrocarbons commonly found in soil and ground water include components of gasoline, such as benzene, toluene, ethylbenzene, and xylene (also referred to as BTEX). Other common contaminants of soil and ground water include naphthalene, chlorinated solvents, and polycyclic aromatic hydrocarbons (PAHs), such as benzopyrenes, created from combustion, coal coking, petroleum refining, and wood-treating operations; nitroaromatic compounds such as trinitrotoluene utilized in production of explosives; inorganic compounds such as metals and cyanides; and polychlorinated biphenyls (PCBs), once widely used in electrical transformers and capacitors and for a variety of other industrial purposes. Because of the widespread use of these and other industrial chemicals, contaminated soil and ground water has been found in many sites around the world.
During the 1990s, as the limitations of conventional subsurface remedial technologies had become increasingly clear, new technologies emerged to restore contaminated soil and ground water. Some of those newer technologies used on contaminated soil and ground water at U.S. Superfund sites include air sparging, bioremediation, passive treatment wall, dual-phase extraction, in situ well aeration, in situ oxidation, and pump and treat methods. However, air sparging, dual-phase extraction, pump and treat methods, passive treatment wall, and in situ well aeration technologies each include high equipment and labor costs with mechanical treatment of ground water. In contrast, bioremediation and intrinsic remediation have exhibited a long-term approach but are still being optimized and have yet to be proven as generally effective, primarily owing to concerns associated with providing an environment optimal for multiplication of the microbes while consuming the contaminant(s).
One of the more conventional technologies for remediation of contaminated ground water is based on the principle that if enough water is pumped from the site, the contaminants will eventually be flushed out. In such “pump and treat” methods, the pumped-out water is treated ex situ to remove contamination, which has limited effectiveness, especially for remediation of undissolved sources of contamination beneath the water table. In addition, several key contaminant and subsurface properties may interfere with flushing include: solubility of contaminants into water; diffusion of contaminant into micropores and zones with limited water mobility; absorption of contaminants to subsurface materials; and heterogeneity of the subsurface. Because of the difficulty of flushing contaminants from the subsurface, the NRC concluded in its 1994 study that pump and treat methods would likely be unable to fully restore many types of contaminated sites.
Other systems are known for oxidizing hydrocarbons to less harmful chemical constituents in situ, both in soil and in ground water. One such oxidizing agent known for such a use is hydrogen peroxide. For example, in the Fenton reaction, hydrogen peroxide can be mixed with a metallic salt such as ferrous sulfate to produce a hydroxyl free radical:H2O2+Fe+2→OH•+OH−+Fe+3 where H2O2 is hydrogen peroxide, Fe+2 is ferrous iron, OH− is hydroxyl free radical, OH− is hydroxyl ion, and Fe+3 is ferric iron. The hydroxyl radical is capable of breaking bonds in certain organic molecules in an exothermic reaction to produce products, including carbon dioxide, water, and other less-hazardous compounds. Particular in situ systems utilizing Fenton-type reactions have been disclosed by Brown (U.S. Pat. No. 4,591,443) and Wilson (U.S. Pat. No. 5,525,008), both of which include mixing the Fenton reactants prior to introduction into the soil and ground water. Vigneri (U.S. Pat. Nos. 5,286,141 and 5,520,483) has described a remediation method and system that includes a pre-acidification of the ground water prior to a sequential introduction of the Fenton reactants, wherein hydrogen peroxide is added after an injection of ferrous sulfate at a high concentration. Yet other Fenton-type systems have been disclosed by Watts et al. (U.S. Pat. No. 5,741,427), Cooper et al. (U.S. Pat. No. 5,967,230), and Whisman, III (U.S. Pat. No. 7,175,770). Such Fenton-based systems are capable of oxidizing a wide range of organic contaminants. The foregoing publications, and each of the subsequent publications cited herein are incorporated herein by reference.
Fenton's reagent chemistry is complex, involving a number of additional reactions producing both oxidants and reductants that contribute to contaminant destruction:OH•+Fe+2→OH−+Fe+3 Fe+3+H2O2→H+HO2•+Fe+2 Fe+2+HO2•→Fe+3+HO2−Fe+3+HO2•→Fe+2+O2+H+OH•+H2O2→H2O+H++O2•−where HO2• is hydroperoxyl radical (a weak oxidant), HO2− is hydroperoxyl anion (a reductant), O2 is molecular oxygen, O2•− is superoxide radical (a reductant), HO2− is hydronium ion, and H2O is water. The suite of reactions associated with the Fenton reaction is generally considered an oxidative system, and hydroxyl radical production is the predominant reaction. Though reductants such as hydroperoxyl anion and superoxide radical may be produced, it has been reported that the amount produced is too low, and the duration over which production occurs is too short to be practically useful.
Another oxidizing agent known to produce oxidizing radicals includes the catalyzation of persulfate ion (S2O8−2), which produces sulfate radicals (SO4•−) and hydroxyl radicals:S2O8−2+Fe+2→SO4•−+SO4−2+Fe+3 SO4•−+OH−→OH•+SO4−2 Particular systems utilizing persulfate-type reactions for in-situ soil and groundwater remediation have been disclosed by Hoag (U.S. Pat. No. 6,019,548), Bruell (U.S. Pat. Appl. No. 2004/0197150), and Block et al. (U.S. Pat. Appl. Nos. 2005/0258110, 2007/0189855, and 2007/0280785). It has been reported that sodium persulfate can also be catalyzed by high pH (>10.5), high concentrations of transition metals such as iron (>150 mg/L), and heat (at acidic or near-neutral pH).
Without being bound by theory, it is believed herein that some sites with certain geological characteristics may consume remediation reactants, such as high-carbonate soils containing high concentrations of limestone or shells, and can act as a buffer by raising the pH of the treating components of conventional systems and reduce their efficacy. In addition, dissolved bicarbonate in alkaline, i.e. hard groundwater, may also act as a radical scavenger, and reduce the efficacy of the treating components of conventional systems.
Yet another oxidizing agent for such use in remediation is permanganate ion (MnO4−). Permanganate, which may be provided as one or more soluble ionic salts, can be mixed with soil and ground water in-situ, and is also capable of breaking bonds by direct reaction with certain organic compounds, primarily unsaturated aliphatic compounds such as trichloroethylene (C2HCl3), as illustrated in the following reaction:2MnO4−+C2HCl3→2MnO2(s)+2CO2+3Cl−H+Particular systems utilizing permanganate-type reactions for in-situ soil and groundwater remediation have been disclosed by Parker et al. (U.S. Pat. No. 6,274,048), Oberle (U.S. Pat. No. 6,315,494), and Cowdery et al. (U.S. Pat. No. 6,869,535).
However, many common environmental contaminants are resistant to chemical oxidation, and instead are more readily destroyed in-situ by chemical reduction. Examples of such contaminants include chloromethanes, such as carbon tetrachloride, chloroform, and methylene chloride; nitrobenzenes; Freons™; PCBs; acetone; and oxidized metals such as hexavalent chromium. Methods are known to produce reductants, such as superoxide radical (O2•−), but such methods are either enzymatic methods, or require aprotic solvents which themselves are often considered environmental contaminants. Further, it has been reported that such methods have limited utility in environmental remediation. Although it is known that Fenton-type and persulfate-type in-situ chemical oxidation systems may also produce small amounts of chemical reductants, such as superoxide radical, the amounts produced have been reported to be insufficient for use in remediation processes. In addition, the superoxide production has been reported to be short-lived and is rapidly terminated.
Accordingly, processes that are capable of generating higher levels of reducing agents, and generating such reducing agents over longer periods of time are needed. Further, such processes are desirably non-enzymatic and/or do not require the concomitant use of aprotic solvents to increase the amount of reducing agents that are generated or the period over which they are generated.
It has been surprisingly discovered that reductants may be generated in higher amounts, and over longer periods of time, in processes that include reacting Mn(IV) with a peroxide, where the reaction is stabilized by addition of either a buffer or compound capable of forming a ligand with manganese, or both.
In one illustrative embodiment of the invention, processes are described herein for decreasing the concentration of contaminants at or in a site by contacting the site with (a) Mn(IV) or a compound capable of generating Mn(IV); (b) a buffered aqueous solution, such as for example a buffered aqueous solution having a pH in the range from about 6 to about 10, or from about 6.5 to about 8, or near neutrality, and the like; and (c) a peroxide, or a compound capable of generating a peroxide. In one variation, the buffered aqueous solution further comprises one or more ligands of a manganese ion and/or one or more compounds capable of forming a ligand with a manganese ion. Such processes may be used to decrease the concentration of a wide variety of contaminants, including highly oxidized carbon contaminants, nitrogen containing contaminants, oxidized metals, other organic and inorganic contaminants, and the like.
In another illustrative embodiment, processes are described herein for decreasing the concentration of contaminants at or in a site by contacting the site with (a) Mn(IV) or a compound capable of generating Mn(IV); (b) one or more ligands of a manganese ion and/or one or more compounds capable of forming a ligand with a manganese ion; and (c) a peroxide, or a compound capable of generating a peroxide. In one variation, the ligands of the manganese ion and/or the compounds capable of forming the ligand with the manganese ion are included in a buffered aqueous solution having a pH in the range from about 6 to about 10, from about 6.5 to about 8, or near neutrality, and the like. Such processes may be used to decrease the concentration of a wide variety of contaminants, including highly oxidized carbon contaminants, nitrogen containing contaminants, oxidized metals, other organic and inorganic contaminants, and the like.
In another embodiment of the processes described herein, at least one of the compounds capable of forming a ligand with manganese is phosphate. In another embodiment of the processes described herein, the buffer comprises phosphate.
It is to be understood that in each of the processes described herein, the source of Mn(IV) may arise from a compound that includes manganese at an oxidation state of (IV), or alternatively manganese at a different oxidation state that is capable of generating Mn(IV) during the performance of the process. For example, it is appreciated that Mn(II) and Mn(III) may oxidize to Mn(IV) under ambient conditions when exposed to oxygen, or other compounds already present at the site, whether contaminants or naturally occurring compounds. It is further appreciated that Mn(VII), such as permanganate, may reduce to Mn(IV) under ambient conditions when exposed to certain transition metals, and/or organic compounds, including contaminants and/or naturally occurring compounds, that are subsequently oxidized.